Since the advent of 2-mercaptobenzothiazole as a rubber vulcanization accelerator, extensive investigation has been conducted to uncover derivatives thereof useful for the same purpose. Among such derivatives which have been found of particular value are various benzothiazyl sulfenamides. In particular, N,N-diisopropylbenzothiazyl-2-sulfenamide has been found to be a particularly outstanding delayed action accelerator for both natural and synthetic rubbers.
Various procedures have been proposed for preparing benzothiazyl-2-sulfenamides in general. For instance, U.S. Pat. No. 2,417,989 teaches the process of producing a sulfenamide by adding an aqueous solution of an oxidizing agent and an aqueous solution of an aryl thiazole, in the form of a water soluble alkali metal mercaptide, separately and concurrently to an aqueous solution of an amine or an amine salt. U.S. Pat. No. 2,419,283 teaches a process for the preparation of sulfenamides wherein an alkali metal sulfate, for example, sodium sulfate is present during the oxidation reaction between an oxidizing agent, a mercaptan and an amine. U.S. Pat. No. 2,776,297 teaches a process for preparing N,N-diisopropylbenzothiazyl-2-sulfenamide by reacting a metallic or ammonium salt of mercaptobenzothiazole with N-chlorodiisopropylamine in an aqueous medium, conducted in the presence of at least a minimum amount of diisopropylamine and under controlled conditions of hydrogen ion concentration. Belgian Pat. No. 897,708 discloses a process for preparing benzothiazyl-2-sulfenamides by reacting sodium mercaptobenzothiazole with an amine in an alcohol/water solution.
Nowhere in the prior art has it been disclosed that N,N-diisopropylbenzothiazyl-2-sulfenamide can be prepared by contacting an aqueous mixture of diisopropylamine and 2-mercaptobenzothiazole with an oxidizing agent in the presence of sufficient acid to neutralize the excess base, if any, in the oxidizing agent.